为什么选择大小写将我的for循环切成两半？

Question

In the following code snippet, tasks is a buffered channel of length 30 that is populated with exactly 30 elements. I'm writing a for loop to operate on each task, which is read in from a channel.

for i := 0; i < len(tasks); i++ {
    fmt.Println(i)
    select {
      case task := <-tasks: 
        fmt.Println(task)
        // Do something

    }
}

fmt.Println("Done")

However, this for loop only runs from 0 to 14. When I vary the length of this channel (which depends on the number of task elements I have), the for loop always only runs for half of len(tasks). Why is this the case?

Background: I use a buffered channel for the tasks because I intend to execute each task in a goroutine, and tasks are dealt with if they fail. But I've currently cut code down to a select case in a for loop, and I'm confused about why the select case causes the for loop to only execute for half the time. I confirm that this for loop has finished executing, and that after the last execution, i is equal to 14 (channel length is 30).

Answer 1

len(task) decreased if you read from tasks

Use empty for to iteratively read all from task.

for {
    select {
    case task := <-tasks:
        fmt.Println(task)
        // Do something
    }
    if len(tasks) == 0 {break}
}

Or alternatively you can use range:

for task := range tasks {

    fmt.Println(task)
    // Do something

    if len(tasks) == 0 {break}
}

Do not forget to break this cycles if you don't want to lock you goroutine.